The present invention relates to a feed-forward amplifier of a type preventing unwanted waves from being outputted in an initial operation of the amplifier, suitable for use in a radio transmitter-receiver in a radio communication system.
In recent years, an amplifier of a feed-forward type (a feed-forward amplifier) has been developed for the purpose of remedy for non-linear distortion appearing in the amplifier. This amplifier, however, could not ensure a sufficient distortion compensating quality until a feed-forward control becomes stable after a start-up of the amplifier, and is not able to sufficiently suppress a pilot signal used for control.
The unwanted waves are contained in an output of the amplifier, causing signal waves at unnecessary frequencies to be outputted. In particular, if the feed-forward amplifier is used as a power amplifier for a radio transmitter-receiver, it is necessary to provide a function to prevent such unwanted waves from being outputted in an initial operation after a start-up of the system until the feed-forward control becomes stable.
A typical feed-forward amplifier includes, for example, a switch disposed at an output of the amplifier functioning to prevent such unwanted waves from being outputted. During an initial operation from a start-up of the amplifier until the feed-forward control becomes stable, the switch is OFF, in order to prevent unwanted waves from being outputted. After the feed-forward control becomes stable, the switch is controlled to be ON to start a transmission.
If such typical feed-forward amplifier is used as a power amplifier in a radio transmitter-receiver, a transmitting output level of the power amplifier is specified by an output of the radio transmitter-receiver. If a switch to prevent unwanted waves from being outputted is interposed somewhere on the side of an output of the amplifier, it is necessary to increase an output level of the amplifier in order to compensate for a power loss caused by the interposition of the switch. The increased output level next, causes an increase in a saturated output level of the amplifier and an increase in power consumption.